1. Field of the Invention
The present invention relates to a process for reducing the oxygen incorporation into a single crystal of silicon, the single crystal being drawn from a melt which is present in a quartz-lined crucible. The invention furthermore relates to an apparatus for carrying out the process.
2. The Prior Art
This method of growing single crystals is known as the Czochralski method. The content of oxygen in single crystals of silicon which have been produced by the Czochralski method has substantial effects on the subsequent use of the single crystal as a basic material for the production of electronic components. Doping the crystal lattice with oxygen causes problems in the region of the thin wafers which is close to the surface, the single crystal initially being divided into said wafers. The electronic structures are integrated in this region, and every impurity not deliberately introduced there may subsequently adversely affect the satisfactory functioning of a component or even lead to the failure of entire circuits. It is therefore ensured, for example, by thermal treatment of the wafer, that oxygen incorporated in the crystal diffuses out of the region close to the surface, resulting in the formation of a zone which has a low oxygen content and typically extends to a depth of 10 .mu.m into the interior of the wafer. In the wafer region below this, which acts only as a substrate for the components, the oxygen incorporated in the crystal forms, in the course of treatment of the wafer, precipitates which, as nucleating centers, also attract and bind the metallic impurities which particularly interfere with the functioning of the components from all regions of the wafer. For this purpose, referred to as "intrinsic gettering", incorporation of oxygen during drawing of the single crystal is entirely desirable. The disadvantage of the Czochralski method is that the incorporation of oxygen during drawing of the single crystal does not take place at a constant rate, so that different oxygen concentrations are measured along the longitudinal axis of the single crystal, which is as a rule rod-like. At the beginning of the crystal growth, when the inner wall of the crucible is substantially covered by the melt, the incorporation of oxygen into the single crystal is the greatest. The concentration of oxygen in the single crystal decreases toward the end of the rod.
According to U.S. Pat. No. 4,545,849, the oxygen incorporation into the single crystal can be increased and the axial distribution of the oxygen in the single crystal can be rendered uniform by means of a toroidal body of silica which is present on the bottom of the crucible during drawing of the crystal. The disadvantage of this is that the wafers produced from such a single crystal require relatively long annealing times for the production of that region close to the surface which has a low oxygen content. The producers of electronic components are therefore increasingly also demanding silicon wafers having lower oxygen concentrations, especially since removal of impurities from those wafer regions close to the surface in which the electronic structures are integrated is possible not only by "intrinsic gettering", but also by a treatment of the back of the wafer which is referred to as "backside damage." However, only single crystals having particularly high oxygen concentrations can be drawn by the process described in U.S. Pat. No. 4,545,849. The rod-like single crystals which are produced by the usual Czochralski method have, particularly at the beginning of the rod, oxygen concentrations which are so high that the wafers produced therefrom often no longer meet the customers' requirements.